extract icmp into its own static library; replace all __attribute__((constructor)) self-registrations with explicit init() calls in dispatch_init

icmp/CMakeLists.txt

@@ -0,0 +1,11 @@
+add_library(icmp STATIC icmp.cpp)
+
+target_include_directories(icmp PUBLIC ${CMAKE_CURRENT_SOURCE_DIR})
+
+target_compile_options(icmp PRIVATE -Wall -Wextra -Wno-unused-parameter)
+
+target_link_libraries(icmp PUBLIC
+    util
+    eth
+    ipv4
+)